On Unity of Function m Organized Bemgs. 93 



necessity of resting their comparison upon the elementary struc- 

 ture of organs, their connections with each other, and the 

 changes they undergo during the progress of their develop, 

 ment. Neither of these grounds of judgment can, I think, 

 safely be trusted to alone ; whilst, combined with each other, 

 they furnish a body of evidence which is quite irresistible. The 

 truth of these observations will, I trust, appear in the sequel ; 

 but I might, in the mean time, adduce in illustration the mode 

 in which the true character of the wings of insects is to be as- 

 certained. 



If we pass in review the various means by which the loco- 

 motive organs of different classes of animals have been placed 

 in relation with the resisting or impelling powers of the atmo- 

 sphere, we shall observe a community of function, and a general 

 similarity of external form, concealing a total difference of in- 

 ternal structure. In most cases, however, we may remark that 

 the wing or other organ of propulsion, however it be constructed, 

 is only a variation from the usual form of a corresponding part 

 in the neighbouring groups ; since " nature, in effecting a new 

 purpose, is inclined to resort to the modification of structures 

 already estabhshed as constituent parts of the frame, in prefer- 

 ence to creating new organs, or such as have no prototype in 

 the model of its formation." The question, therefore, naturally 

 arises with regard to the wings of insects, whether they are to 

 be considered as new organs, superadded to those which are 

 found in the adjoining classes, and in the early stages of their 

 own existence, or whether they can be shewn to be the result 

 of an extension or increase of development on the part of some 

 structure ah-eady present, although perhaps assuming a differ- 

 ent form. We shall then first inquire what inference may be 

 drawn respecting the real nature of the wings of insects from 

 their anatomical structure. They may be readily shewn to 

 consist of a fold of external membrane, extended upon ribs or 

 nerves, which are principally formed of trachea connected with 

 those in the interior of the body. It is only recently that the 

 circulation of fluid has been observed ia the wings. Carus de- 

 scribes it as visible in the pupa of many species, but he does 

 not seem to have detected it in more than a few cases after tho 

 last metamorphosis. My friend Mr Tyrrell of Exeter, in- 



